Preheat the oven to 180C.
In a large saucepan, fry the leeks and thyme in oil until softened.
Add the mushrooms and chicken and cook until the chicken is browned.
Add 50g flour and stir well to combine.
Season with salt and pepper and stir in 250ml chicken stock and 150ml dairy free cream alternative.
Pour the mixture into a pie dish.
Roll out the pastry and place on top of the pie. Cut to fit with scissors - remember the pastry will shrink as it cooks so be generous. Cut a cross in the middle and brush with the beaten egg, if using.
Bake in the oven for 30-40 minutes, until the pastry is golden.